(1) Field of the Invention
The present invention is a comparison calibration of hydrophone data channels in a towed line array of hydrophones in which the comparison calibration extends a low frequency limit that is applicable to existing calibration methods that seek to approximate free field propagation in open water.
(2) Description of the Prior Art
Among the principle challenges in the calibration of a hydrophone towed line array is the large size of the array itself. Hydrophone line arrays can have lengths of hundreds (or thousands) of meters. Thus, acoustic calibration of hydrophone line arrays under controlled laboratory conditions is not generally practiced (nor feasible).
Calibration of a towed hydrophone line array is typically performed using a comparison method. When calibrating, one of more reference standard hydrophones with a known free field voltage sensitivity (FFVS) are used to measure an acoustic field that is transmitted by an acoustic projector at one or more locations in the vicinity of a test article (the towed hydrophone line array). The FFVS is a complex quantity with a magnitude and a phase. The magnitude is traditionally expressed as decibels relative to one volt per micro Pascal (e.g., dB re 1 V/uPa) and the phase is expressed in degrees.
A problem when using acoustic free field or gated continuous wave calibration techniques for towed line arrays is the low frequency limit that is imposed by a requirement for reflection free signals. Various methods have been developed to calculate the FFVS of large numbers of hydrophones distributed over the length of the towed line arrays [See L. J. Hix “Method and Apparatus for Testing Marine Seismic Cables” U.S. Pat. No. 4,160,228].
The Hix reference discloses a method by which the array to be calibrated is towed past a stationary acoustic projector in an open body of water. The acoustic projector transmits acoustic signals with known properties. The electrical outputs of the array hydrophones are recorded on the tow ship where individual channel outputs are compared with one another. Since the channel outputs are not compared to a calibrated reference standard; the FFVS of the array channels are not provided. This practice is often referred to as a relative calibration.
In Skinner (“Place Calibration of Sonar Receive Array”; U.S. Pat. No. 6,208,584), a method is disclosed whereby both a test article and an acoustic projector are towed behind a ship in an open body of water. The method compares the magnitude and phase of the hydrophone channel outputs in the test article. However, an independent reference standard hydrophone is not used; therefore, the FFVS is not provided by this relative calibration method.
Percy (“Hydrophone Line Array Calibration Unit”, U.S. Pat. No. 3,959,620) teaches a method by which a hydrophone line array is affixed to a cylindrically shaped wire mesh framework. One calibrated reference standard hydrophone is also affixed to the framework to provide an absolute reference for comparison. This practice is often referred to as an absolute calibration.
The Percy reference includes an acoustic projector located at an approximate geometric center of the cylindrical test fixture. The acoustic projector is used to transmit acoustic interrogation signals to a test article and reference hydrophone. The cited reference does not teach a method of support for the acoustic projector to prevent the transmission of vibration into the cylindrical test fixture, array and reference standard hydrophone. Such vibrations tend to corrupt the received acoustic data; thus, introducing both random and systematic errors into the calibration result.
The Percy reference also assumes the existence of an acoustically non-reflective test tank in which the calibration is performed over a frequency range of 10 to 1000 Hz. The cited reference does not teach a method to create acoustically non-reflective test tank surfaces, nor is a method taught to render the water surface to be acoustically non-reflective. Acoustic test tanks with the non-reflective properties assumed by the Percy reference are not known in the prior art. Thus, acoustic data will likely contain contributions from reflected acoustic energy with commensurate errors in the FFVS computed for the array hydrophone channels.
Most acoustic calibration procedures, including those taught by the Hix, Skinner, and Percy references; tacitly assume that the properties of the acoustic field are known to arbitrary precision. Indeed, this is typically the case for calibration tests conducted in the highly controlled environment of an enclosed laboratory. However, this is rarely the case for calibrations performed in open water where boundary reflections may be unavoidable.
When performing acoustic measurement in an open body of water (such as lakes, ponds and pools used by acoustic test facilities); there exists a minimum frequency at which free field propagation can be simulated by appropriate gating of the time series signals such that reflections from the boundaries (surface, bottom and sides) can be excluded. Above this limiting frequency, a time gated acoustic waveform can be transmitted and received on the reference standard hydrophones and the array hydrophones (test article) under approximately free field conditions.
Reflections from the surface or other boundaries are eliminated by appropriate time gating of the received signals. In this frequency band, calibration of the test article can be performed using techniques taught by the prior art [See “American National Standard: Procedures for Calibration of Underwater Electroacoustic Transducers”, ANSI/ASA S1.20-2012].
As discussed previously, this method has a low frequency limit governed by the requirement to approximate free field conditions. At lower frequencies, it is not possible to collect time series data of sufficient length without including acoustic field components that are reflected from the water surface or other boundaries.